1. Field of the Invention
The present invention relates to a liquid ejection apparatus and a drive method for an inkjet head, and more particularly to driving technology for a piezoelectric device arranged in an inkjet head.
2. Description of the Related Art
As drive methods for inkjet heads in inkjet recording apparatuses, there are known a piezoelectric method and a thermal method. In the piezoelectric method, ink is ejected from a nozzle by using a mechanical distortion of a piezoelectric device. In the thermal method, ink is ejected from a nozzle by using a film boiling effect of ink stored in a liquid chamber connected to the nozzle. The piezoelectric method has a beneficial effect in that the ink ejection volume and ejection velocity can be controlled more readily than the thermal method.
The piezoelectric device used in the inkjet head needs to be constituted of a piezoelectric element made of material having a high piezoelectric coefficient (electrical-mechanical transducing coefficient). For example, known materials for the piezoelectric elements include lead titanate (PT), lead zirconate titanate (PZT), and PZT doped with magnesium, manganese, cobalt, iron, nickel, niobium, scandium, tantalum, bismuth, or the like. The piezoelectric element used in the inkjet head is applied with an electric field of about several kilovolts per centimeter, in order to generate the pressure required for the ejection of ink.
Here, it is known that piezoelectric elements have a large number of defects, such as small cracks or voids, and the like. When an electric field of high intensity is applied to a piezoelectric element containing lead under conditions where water (moisture) is present, large electric current flows through the lead compound in the defective portion and the peripheral area thereof, and the location where the large electric current occurs is destroyed by Joule heat and hence creates a larger defect.
By forming the piezoelectric element with a large thickness, it is possible to avoid the occurrence of large defects which pass through the element due to the breakage; however, it becomes necessary to apply an electric field of high intensity to the piezoelectric element in order to obtain a desired pressure applied to the liquid in the liquid chamber, and hence there are concerns about increase in the electric power consumption.
Japanese Patent Application Publication No. 2004-322605 discloses an inkjet recording apparatus which is provided with a dew point control unit so as to keep the dew point in the atmosphere around the piezoelectric element and the vicinity of the piezoelectric element lower than the dew point in the environment of the inkjet recording apparatus, and describes that it is thereby possible to achieve a thin thickness of the piezoelectric element while preventing the breakage of the piezoelectric element due to application of high voltage to the piezoelectric element. More specifically, the dew point control unit includes a compressor and an air drier, which dries air compressed by the computer and supplies the dried air to a casing in which the piezoelectric element is sealed. However, in this composition, when the supply of the dried air to the casing is stopped, moisture can flow back into the casing through an inlet of the dried air and an outlet of the overflowing air. Moreover, if the atmosphere around the piezoelectric element is of high humidity before starting the supply of the dried air, then there is a problem of breakage of the piezoelectric element occurring the moment a voltage is applied to the piezoelectric element.